Aviat Space Envir Md
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Aviat Space Envir Md · Jan 1992
Effects of 1-week head-down tilt bed rest on bone formation and the calcium endocrine system.
To understand the potential early responses of human bone and the calcium endocrine system to spaceflight, we studied 8 healthy men, aged 35-44 years before, during, and after bed rest in a -6 degrees head-down tilt model for microgravity. Based on a novel single-dose labeling schedule, average rates of bone formation in the iliac crest were reduced in 6, unchanged in 1, and increased in 1 following the bed rest period. The decrease was greatest for subjects whose daily walking miles were highest (r = -0.762, p less than 0.05, n = 7). ⋯ Reduced excretion of phosphorus and 3', 5' cyclic adenosine monophosphate on the first and second bed rest days was followed by an increase in serum phosphorus by the sixth bed rest day. Depressed serum concentrations of parathyroid hormone and 1,25-dihydroxyvitamin D were manifest by the sixth and seventh bed rest days. The similarity of the response of bone and the calcium endocrine system of healthy men after only 7 days to results of longer term bed rest studies emphasizes the responsiveness of the adult human skeleton to biomechanical stimuli induced by changes in activity and/or position.
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Aviat Space Envir Md · Dec 1991
Cardiopulmonary responses to acute hypoxia, head-down tilt and fluid loading in anesthetized dogs.
The separate and combined acute effects of hypoxia (HY-11% O2), head-down tilt (HD-30 degrees) and fluid loading (FL-1.0 L saline) on hemodynamics and pulmonary gas exchange were determined in 17 anesthetized, mechanically ventilated dogs. Both during HY and normoxia (NO), the total respiratory compliance was decreased by HD, attributable to pulmonary vascular congestion. The reductions in compliance were twice as great with FL, indicating pulmonary interstitial edema, which was supported by histological observation of lung tissue. ⋯ A significant positive correlation between the changes in blood volume and pulmonary artery pressure for experimental transitions suggests that a shift in blood volume from systemic to pulmonary circulations and changes in total blood volume probably contributed substantially to these apparent changes in resistance. Pulmonary gas exchange efficiency, whether expressed in terms of shunt or ventilation/perfusion distribution from arterial-end-tidal PCO2 and PO2 differences, showed a significant inverse relationship with pulmonary driving pressure for the experimental conditions imposed. No clear synergistic effects of HY on HD were evident in contributing to pulmonary edema when superimposed prior to FL, but after FL this risk must be considered.
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During his term as surgeon general, Maj. Gen. Harry G. ⋯ A. F. Armstrong Laboratory.
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Aviat Space Envir Md · May 1991
Heat balance of subjects wearing protective clothing with a liquid- or air-cooled vest.
The goals of this study were, first, to determine the extent of the heat strain induced by wearing the Canadian Forces (CF) aircrew chemical defence individual protection ensemble (CD IPE) under simulated hot cockpit conditions, and second, to determine the effectiveness of a liquid cooled (LC) and an air-cooled (AC) vest in relieving such heat strain. Seven (7) healthy male subjects were subjected to three heat exposures (37 degrees C, 50% r.h., for 150 min, time-weighted metabolic rate of about 240 W, 1 week apart) either with no cooling (NC), LC or AC vests. NC was only tolerated for 95 +/- 5 min, whereas all subjects completed the 150-min tests with AC or LC (p less than 0.01). ⋯ NC entailed a sweat rate of almost 1 kg/h, which was reduced 38% by LC and 51% by AC (p less than 0.01). The combined dry and evaporative heat losses (HEKC of LC and AC vests were significantly greater than that of NC (164 +/- 7 and 181 +/- 9 vs. 124 +/- 9 W, respectively; p less than 0.01). The results demonstrate that subjects wearing CF aircrew IPE under simulated hot cockpit conditions can only tolerate 95 min of the 150-min test, and experience significant heat strain.(ABSTRACT TRUNCATED AT 250 WORDS)
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Aviat Space Envir Md · Feb 1991
Comparative StudyCardiac output by impedance cardiography: two alternative methodologies compared with thermodilution.
One critical component of the Kubicek (8) stroke volume equation, used to measure cardiac output by impedance cardiography, is dZ/dt(max). The present study, conducted on six anesthetized dogs, compared impedance cardiac output derived using two alternative methods of quantifying dZ/dt(max), with simultaneous thermodilution measures. Values for dZ/dt(max) quantified relative to dZ/dt = 0 baseline and relative to the dZ/dt B-point were entered into the Kubicek equation to generate cardiac output measures designated as impedance-A and impedance-B, respectively. ⋯ For the impedance-B method, similarity to thermodilution was limited to two of the three drug conditions. Correlations of impedance cardiac output with thermodilution tended to be higher for the impedance-A method. Together with theoretical and practical considerations, these results support quantification of dZ/dt(max) relative to dZ/dt = 0 when used in the Kubicek stroke volume equation.